Intermediate Sobolev norms can be defined
in terms of powers of the operator \(\Lambda \)  as in
[1]} (Section 2.1). We denote these spaces by \(W^{\epsilon }(U)\) :
\(W^{\epsilon }(U) = \mbox{dom}(\Lambda ^{\epsilon })\)  for
\(0\le \epsilon \le 1\) , with norm
\(\Vert  f\Vert _{W^{\epsilon }(U)} = \Vert f\Vert _{L^2(U)} + \Vert \Lambda ^{\epsilon } f\Vert _{L^2(U)} .\) 
